I. Field of the Invention
This invention relates to servo control of a pair of rotatably driven spools for positioning a tape web between the spools. More particularly, the present invention relates to such servo control for winding and unwinding the web from the spools to accurately apply strips or courses of tape carried by the web to a contoured surface.
II. Description of the Prior Art
By way of background, a computerized tape-laying machine may include a tap dispensing apparatus which is positionable and movable under computer control relative to a layup tool or the like to which multiple strips or courses of composite tape material are to be applied to form parts such as aircraft wings, for example. A length of composite tape adhered to a tape backing web (also referred to as a substrate or backup paper) is dispensed by a tape applicator head including a pair of spools. Tape is generally supplied from one of the spools referred to as a feed reel and the web is generally collected on the other spool referred to as a takeup reel. Intermediate the spools is a tape application member or shoe across which the web is to pass on its traverse between the spools and whereat tape is to be peeled from the web and adhered to the layup tool. In cooperation with movement of various machine members, the tape dispensing shoe of the tape applicator head is movable in a plurality of rectilinear and/or rotational axes under control of a computer program by which to apply several plies of tape to the layup tool placed between the pylons.
For example, the tape applicator head is movable vertically upwardly and downwardly relative the layup tool by a Z-axis servo control under program control of a computer. Also, the tape applicator head is movable horizontally relative the layup tool in an X-axis (by movement, for example, of a gantry) and in a Y-axis (by movement, for example, of a carriage) all under program control. The X-, Y-, and Z-axes are orthogonal. The tape applicator head is further movable under program control rotationally and along an arc over the layup tool, for example, along C- and A-axes, respectively. The C-axis is parallel to the Z-axis. The A-axis is perpendicular to the Z-axis. Finally, the web itself is movable between the spools and over the tape dispensing shoe along a U-axis.
Movement of the web in the U-axis has typically been accomplished by frictional engagement of the tape with the layup tool. This may be referred to as "torque mode." In the torque mode, counterrotating forces are applied to the spools. As is conventional, each spool is coupled to the rotatable shaft of a servo motor or drive. As is also conventional, commands from a computer control or the like to a servo control will cause the servo control to generate appropriate voltage signals to effectuate rotation of the servo motors. In the aforesaid torque mode, each spool is to be rotated by its associated servo motor in a direction which would wind up the web onto the spool. However, because each spool is attempting to wind up the web, the web is placed under tension between each spool and the shoe. Importantly, the web is also kept under tension as it passes by tape cutters situated between the shoe and feed reel whereby to cut the tape (but not the web) to the desired lengths and along the appropriate angle across the width of the tape to define the header and tail ends of the tape strips.
As is well understood, the torque mode of control is open loop such that voltage signals from the servo control to the spool motors are not varied depending upon performance of the servo motors. The specific voltage signals are correlated to the desired tension at the spools as determined by an operator. For this reason, the voltage signals utilized in the torque mode may be seen to comprise "static torque" signals.
To apply tape, the shoe is moved towards the layup tool until it compacts the tape against the layup tool (on-plane). Typically, tape is applied in strips or courses beginning at a header and ending at a tail of the strip. At the beginning of the application of each strip, the forward edge of the header may be situated below the shoe and upon movement of the tape head, frictional forces would tend to peel the header from the web and adhere it to the layup tool. The frictional forces also tend to move the web in the U-axis. The slackening or tightening of the web at the respective spool caused by movement of the web is reacted to as a consequence of the static torque signal to the servo motors to maintain web tension. Thus, the takeup servo motor will cause spent web to be wound onto the takeup spool rather than accumulate between the shoe and takeup reel. Similarly, tightening of the web between the shoe and feed reel will tend to overcome the torque at the feed reel causing fresh web to be unwound from the feed reel. Reverse movement of the tape head would cause the reverse to occur (i.e., wind up on the feed reel and unwinding from the takeup reel).
The head continues to move while on-plane until the end or tail of the strip is placed. In some situations, as the tail of the strip is approached, the shoe may be lifted away from the layup tool causing the web to back off from the layup tool as well. A compaction roller may be positioned between the web and tape to cause the tail of the tape to come away from the web and adhere to the layup tool. When the web is spaced away from the layup tool, such as when the shoe is lifted from the layup tool (offplane), there would be no movement of the web in the U-axis as the tension on the web to either side of the shoe would tend to equalize. However, when a compaction roller is used, there may be a need to continue movement of the web across the shoe such as to payoff the remaining tail of the current strip. To obtain such web movement, the machine may include a "position mode" of control in which the takeup reel is provided not only a static torque signal but is further caused to rotate under program control with a variable change in position signal as is conventionally employed to effectuate movement of the other machine axes, i.e., the change in position signals effect driving movement of the web in the U-axis when off-plane. The feed reel, however, continues to operate in the torque mode whereby the torque applied to the feed reel will cause the web to wind or unwind therefrom under tension depending upon direction of rotation of the takeup reel. As is well understood, unlike application of static torque signals, which is an open loop form of control, position mode control is closed loop.
The position mode has also been utilized to reposition the web after tail application. As mentioned, the web may need to be moved across the shoe to payoff the tail of the current strip. However, the beginning or header of the next or adjacent strip of tape should not at this time be removed from the web. When the tail is placed, therefore, part of the header of the next strip may have already passed the shoe on its way to the takeup reel. The position mode may be utilized to reposition the web so that the header of the next strip is under the shoe for subsequent application.
In the position mode of control, the static torque signals to the takeup reel are augmented or replaced with velocity command signals. As is well known, the computer calculates the distance S the tape is to be moved and, based upon predetermined feed rates, determines how far the tape should move over a predetermined time or interpolation interval. The servo control generates a voltage signal corresponding to the desired velocity of the tape web, which velocity is correlated to the change in position commands from the computer.
Also, as is conventional, a roller resolver riding on the web wound onto the takeup reel generates a resolver signal which is utilized to measure longitudinal movement of the web. Coupled between the motor and the servo control is a drive amplifier to supply motor drive currents in response to the voltage signal from the servo control. The motor may also provide a tachometer signal for use by the drive amplifier in a velocity feedback loop as is conventional.
As mentioned, position control is closed loop. Thus, the servo control generates the velocity command signal based upon a following error signal which is typically the difference between the actual extent of tape travel and the desired extent of travel thereof (as calculated using the change in position command signal from the computer). The velocity command signals are then converted in the servo control to voltage signals and coupled through the drive amplifier to the motor to cause rotation of the takeup reel, at a velocity correlated to the following error by a gain factor signal. The gain factor signal is typically selected so that the voltage signal corresponding to the driving command signal will result in movement of the tape at a predetermined velocity correlated to a predetermined following error signal, e.g., 1 inch/min for one-thousandth inch following error signal (1 inch/min per 1/1000 FE).
In summary, "torque-mode" of operation of the U-axis is open loop by coupling static torque signals to the motors driving the spools whereby to maintain the tape web under tension throughout its entire traverse from spool to spool. Thus, with the shoe on-plane, only the frictional forces between the tape and layup tool are relied upon to peel the tape from the web and adhere it to the layup tool as the tape applicator head traverses the layup tool. In the position mode, such as may be utilized to reposition the web after a tail has been applied by compaction rollers, the takeup reel drive is additionally or alternatively responsive to closed loop velocity command signals.
The above operation, however, is not believed to provide sufficiently accurate web placement or positioning information in all cases. For example, stretch of the web cannot be accurately controlled or predicted, thus introducing error into the calculations of the velocity command signals. Similarly, eccentricities of the web on the takeup reel introduce further error.
Additionally, operation as above described is believed to allow for end placement errors of the beginning (header) and end (tail) of any given strip of tape. By way of example, the frontal edge of the header may not be perpendicular the U-axis (i.e., the tape longitudinal axis) but may be angled relative thereto. In that case, the forwardly edge of the header may be a very small widthwise portion of the strip of tape. The shoe will thus initially be overlying only a small portion of the header and as the tape applicator head begins to move in order to apply the tape, the tape may slide somewhat across the layup tool rather than just simply payoff the web. The sliding in some cases may be as much as one-half inch. In this situation, the header of the tape will actually be applied to the layup tool at a position other than desired leading to misapplication of the tape. While use of position mode control has been attempted for laying the header, it is believed that slippage has not been sufficiently reduced. Thus, further improvements in position mode control of the tape web are desired.